1. Field of the Invention
The present invention relates in general to a method and apparatus for determining a geographical location of a mobile unit and more specifically to a method and apparatus for determining a geographical location of a mobile unit using an apparatus traveling above the surface of the Earth.
2. Description of the Related Art Communication systems and techniques for determining the location of a mobile unit are known. Many techniques utilize Global Positioning Systems (GPS) which allow a mobile unit to determine its position based on signals from at least three satellites. GPS systems work well when the mobile unit is in an open area such on an ocean or in the air. However, GPS systems are limited in that the mobile unit must have unobstructed paths to at least three GPS satellites. Some GPS systems require a minimum of four satellites to determine the location of a mobile unit. If substantially clear signals cannot be received from the required satellites by the mobile unit, the mobile unit cannot determine its position.
Techniques have been proposed that utilize Low Earth Orbit (LEO) satellite systems. Satellites in LEO systems orbit at a lower altitude than GPS satellites and are not geostationary but rather orbit the Earth at a high speed relative to GPS satellites. LEO satellite communication systems contain many more satellites than GPS systems in order to attain global coverage at a lower altitude.
At least one proposed technique for determining the position of a mobile unit, such as described in U.S. Pat. No. 5,610,615, attempts to take advantage of the LEO satellite's lower orbit and higher speed. This technique allows a mobile unit to determine its position by obtaining a signal from one satellite and determining an instantaneous elevation angle, angles related to the orbit track of the satellite and other parameters. However, this technique is disadvantaged in that calculations are complex requiring higher production and maintenance costs of the system. Further, the technique requires that the mobile unit process information and perform calculations, adding complexity, size and cost to mobile units. In addition to the disadvantages discussed above, the positioning method described in the references patent does not allow for an accuracy suitable for locating a mobile unit in emergency situations.
Systems requiring the mobile unit to determine its location are further disadvantaged in emergency situations. In emergency situations, a user may not be able to forward location information to emergency services. For example, a user may be losing consciousness or have limited communication abilities due to physical trauma experienced in an accident.
Therefore, there exists a need for a method and apparatus for simply and efficiently locating a mobile user and communicating the location to emergency service providers in times of emergency.